Neurotransmitters are small molecules that carry information across a synapse from the end of one nerve cell to the end of another nerve cell. The neurotransmitters are a diverse group of chemical compounds ranging from simple amines to amino acids to polypeptides. The mechanisms by which they elicit responses in both presynaptic and postsynaptic neurons are similarly diverse. Much research has been conducted on monoamine neurotransmitters such as serotonin, norepinephirine, and dopamine. These neurotransmitters are believed to have important effects on mood and behavior.
Dysregulations in monoamine neurotransmitter systems are currently believed to play a seminal role in the neurobiological underpinnings of certain neuropsychiatric disorders, including depression, anxiety, schizophrenia, eating disorders, Parkinson's disease and sleeping disorders. The actions of these monoamine neurotransmitters are typically terminated by transport (also know as reuptake) of the transmitter back into the cell of origin via cell surface proteins known as transporters.
Treatment of neuropsychiatric disorders often involves manipulation of neurotransmitter transport systems with pharmaceutical agents that inhibit or antagonize a particular transporter. Application of an appropriate antagonist to block uptake prolongs and enhances the action of the neurotransmitter. For example, the serotonin and norepinephrine reuptake blockers, paroxetine and desipramine, respectively, are used as antidepressants.
Determination of the amount of transporter antagonism as a function of drug dose or blood concentration is most successfully studied using in vivo techniques such as positron emission tomography (PET). However, PET is a very invasive procedure that utilizes radioactive isotopes injected into the body and requires access to nearby state of the art research facilities. Because it is undesirable and difficult to use in vivo studies, the effectiveness of new medications or of particular doses of known medications are typically determined empirically by treating human patients and trying to observe discrete measures of therapeutic effectiveness as a function of drug dosage.
The potency of individual drugs to antagonize monoamine transporters can be determined in vitro using cultured cells. In such tests, cells are cultured that contain the transporters corresponding to the neurotransmitters under study. The cells are stabilized in a buffer solution appropriate for maintaining the vitality of the cells. A radioactive form of the neurotransmitter and the proposed inhibitor are added to the buffer, and transport of the radiolabeled neurotransmitter is monitored. Transport with and without the presence of various concentrations of the proposed inhibitor are compared and an overall percentage of inhibition is determined.
Traditional in vitro methods of studying neurotransmitter transport have heretofore provided general insight into the understanding of how antidepressants inhibit neurotransmitter transporters and how potent a given agent may be, but the traditional studies are often inaccurate and based upon vague correlations between overall serum drug concentrations and the predicted transport blockage provided by the drugs in the body.
Individual variations in response to treatment with transport inhibiting antidepressant drugs is little studied and not well understood. Often, to compensate for individual responses, drugs are, as noted above, simply administered to an individual in varying combinations and dosages until the most significant reduction in symptoms are observed. Such methods of determining optimum dosages are uncertain, take long periods of time, and often result in a patient being over or under-medicated for extended lengths of time.
It is desired to provide an in vitro method of studying the effectiveness of monoamine transporter inhibitors that is patient specific and that provides data useful in determining the most effective dose of inhibitor or combination of inhibitors for treatment of a neurological disorder. Such a technique should be minimally invasive and should provide an objective guidance in the determination of optimum drug combinations and dosages.